Increased susceptibility of the left ventricular subendocardium to ischemic injury has been demonstrated during a variety of physiologic maneuvers in both normal and hypertrophied hearts. Other studies of regional myocardial blood flow in hearts with a critical coronary artery stenosis has documented even greater degrees of subendocardial ischemia distal to the stenosis. Studies were conducted on hearts with left ventricular hypertrophy (LVH) secondary to chronically elevated afterload distal to the coronary ostia. The lack of a laboratory model of LVH secondary to aortic valve or subvalve stenosis has hindered complete investigation of regional myocardial blood flow in these settings. Subsequently, no cooperative study has been reported regarding regional myocardial blood flow in hearts with LVH secondary to supra-aortic versus subaortic valve outflow obstruction. To further elucidate this, we are subjecting foxhound dogs with LVH secondary to ascending aortic construction and Newfoundland dogs with LVH secondary to congenital discrete subaortic stenosis to severe physiologic maneuvers after placing a critical stenosis on one major coronary artery. Regional myocardial blood flow is then measured using 9 plus microns radioactive microspheres.